Several defect inspection systems are known in the art, typically relying on a single-spot scan of a sample surface. Scanning the sample with a single illumination beam is appropriate in many applications. However, emerging technologies constantly require higher sampling rates and defect sensitivity than single-spot inspections systems are capable of providing. Accordingly, multi-spot inspection systems have been proposed to improve throughput and sensitivity by substantially simultaneously scanning the sample with a plurality of illumination beams and processing the resulting scan information in parallel.
The multi-spot inspection systems currently known to the art suffer from various complexities. For example, pitch error is a prevalent problem is multi-spot scans of a spinning wafer. Radially spaced spot patterns are less prone to pitch error because a tilt angle between spot path and scan direction is avoided. However, radial spot patterns present are difficult to generate and will typically require a much more complex optical designs than those required for generating tangential spot patterns.